linux-kernel - Re: [PATCH 10/17] prmem: documentation

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Message-ID: <20181031094506.GM744@hirez.programming.kicks-ass.net>
Date:   Wed, 31 Oct 2018 10:45:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Igor Stoppa <igor.stoppa@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Tycho Andersen <tycho@...ho.ws>,
        Kees Cook <keescook@...omium.org>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Dave Chinner <david@...morbit.com>,
        James Morris <jmorris@...ei.org>,
        Michal Hocko <mhocko@...nel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Igor Stoppa <igor.stoppa@...wei.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Laura Abbott <labbott@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 10/17] prmem: documentation

On Tue, Oct 30, 2018 at 02:02:12PM -0700, Andy Lutomirski wrote:
> But I dislike allowing regular writes in the protected region. We
> really only need four write primitives:
> 
> 1. Just write one value.  Call at any time (except NMI).
> 
> 2. Just copy some bytes. Same as (1) but any number of bytes.

Given the !preempt/!IRQ contraints I'd certainly put an upper limit on
the number of bytes there.

> 3,4: Same as 1 and 2 but must be called inside a special rare write
> region. This is purely an optimization.
> 
> Actually getting a modifiable pointer should be disallowed for two
> reasons:
> 
> 1. Some architectures may want to use a special
> write-different-address-space operation.

You're thinking of s390 ? :-)

> Heck, x86 could, too: make
> the actual offset be a secret and shove the offset into FSBASE or
> similar. Then %fs-prefixed writes would do the rare writes.

> 2. Alternatively, x86 could set the U bit. Then the actual writes
> would use the uaccess helpers, giving extra protection via SMAP.

Cute, and yes, something like that would be nice.

> We don’t really want a situation where an unchecked pointer in the
> rare write region completely defeats the mechanism.

Agreed.